1. Field of the Invention
This invention relates to receivers, and, more particularly, to methods and apparatus for recovering information symbols from a plurality of signals.
2. Description of the Related Art
In many types of communications systems, it is desirable to employ antenna arrays to receive signals. Such arrays provide superior performance since they allow for taking advantage of the directionality of received signals. For example, if a basestation is receiving signals from two mobiles with differing angles of arrival, a method may be devised to effectively increase the antenna response in the direction of a desired signal, while decreasing the response in the direction of an undesired interferer. The use of this spatial information to allow greater network capacity is commonly called Space Division Multiple Access (SDMA).
There are two basic methods for processing information from multiple antennae (xe2x80x9cantenna trackingxe2x80x9d). One method involves simply determining which of the antennae is receiving the xe2x80x9cbestxe2x80x9d signal and discarding the other signals. Although this method is relatively easy to implement, it discards information and may thus not be optimal. The second method involves forming some combination of the signals received by the multiple antennas (hereafter the xe2x80x9ccombination approachxe2x80x9d). This method generally provides better results than the previously discussed method but is more difficult to implement.
In direct sequence Code Division Multiple Access (xe2x80x9cCDMAxe2x80x9d) systems (xe2x80x9cDSCDMAxe2x80x9d) with multiple antennae, implementing the combination approach for a basestation is further complicated because the received signals are used for multiple purposes. For example, a basestation actively demodulates several mobile station signals, all of which occupy the same frequency spectrum and are present in the same set of antenna signals. Further, some systems use multiple basestations simultaneously transmitting the same data to the mobile (xe2x80x9csoft handoffxe2x80x9d), so that the signal from each basestation is present in the set of antenna signals, and each basestation""s signal must first be processed independently. Finally, DS-CDMA receivers are subject to the typical multipath problem in which several copies of the same signal arrive at the receiver through different paths.
In addition to the complications arising from multiple use, the implementation of the combination approach is further complicated by the ever increasing need for antenna tracking to be performed quickly. Receivers generally should make antenna tracking updates at a rate given by the signal bandwidth; thus, updates generally must occur more frequently as the bandwidth of a system increases. Therefore, it would be desirable to improve upon the speed of conventional antenna tracking systems, especially as bandwidth requirements continue to increase.
One conventional combination approach for performing antenna tracking involves the use of Wiener filtering. Wiener filtering, including LMS (xe2x80x9cLeast Mean Squarexe2x80x9d) or RLS (xe2x80x9cRecursive Least Squaresxe2x80x9d), is commonly used to implement the combination approach because it is frequently superior to other methods such as beamsteering. However, conventional Wiener filtering combination approaches are subject to certain limitations.
Specifically, in receivers that employ conventional Wiener filtering combination approaches, the receiver""s timing acquisition may not be able to take advantage of multiple antennae. This inability occurs because a receiver must acquire its timing over a certain interval, and antenna tracking cannot be performed until this acquisition is complete. Thus, in such systems, timing acquisition can not be implemented with a combination approach. It would be desirable to use the more accurate combination approach in connection with acquisition of signal timing.
A similar xe2x80x9cchicken and eggxe2x80x9d problem for receivers that employ conventional Wiener filtering may occur in the case of multipath signals (i.e. multiple copies of the same signal arriving at a receiver) being tracked from a single transmitter. In such instances, typically some finite number of fingers are assigned to demodulate each of several different received time delays. As the channel characteristics change, these finger assignments must change. Searching for these delays is hampered analogously to the timing acquisition process discussed above: the combination approach can not converge to a solution until a timing is selected, implying the timing must in some sense be selected without the advantage of the antenna tracking.
CDMA receivers that implement the combination approach with Wiener filtering have still further limitations. In such receivers, the weights that are applied to the different antenna signals before those signals are combined must be calculated at the xe2x80x9cchipxe2x80x9d rate, which is a multiple of the symbol rate. It would be desirable to perform antenna tracking at the lower symbol rate (i.e. after despreading has occurred), thus reducing hardware requirements.
CDMA receivers that are responsible for multiple users (e.g. a basestation) confront still further difficulties in connection with implementing the combination approach. One way to implement the above approach using Wiener filtering in a DSCDMA receiver would be to perform LMS or RLS for each finger for which the receiver is responsible. (There may be multiple fingers per user, as will be further described in the Detailed Description of the Invention.) However, this would be quite computationally intensive; given the previously described constraints on DS-CDMA receivers, this may make using LMS or RLS difficult. On the other hand, as previously mentioned, the combination approach is superior to an approach that simply selects one of many signals that arrive at a receiver.
Thus, there is a need for a DS-CDMA receiver with an antenna array that can efficiently implement the combination approach for multiple users (or multiple receive paths). More broadly, there is a need for a receiver that can implement the combination approach with reduced computational complexity.
These and other needs are met by the present invention. According to an embodiment of the invention, a DS-CDMA receiver receives an input signal which comprises a plurality of received signals that are received over a corresponding plurality of antennae. These signals are demodulated and sampled to create digital signals, which may be represented by a vector ubefore[n], which has one element for each of the plurality of input signals. The vector ubefore[n] is decorrelated by multiplying it with a decorrelation matrix to create a vector uafter[n] which thus represents a modified version of the input signal. The decorrelation matrix is generated by performing certain operations derived from standard xe2x80x9csquare rootxe2x80x9d RLS methodology. A weight vector that is specific to each finger is derived, and each finger""s weight vector is multiplied by the vector uafter[n] to recover a finger""s estimate of a user""s information symbol. Finally, the estimates for each finger corresponding to a particular user are added to derive an estimate of such user""s information symbol. The weight vector for each finger is based upon a cross correlation between the decorrelated received signals, represented by uafter[n], and the desired signal. According to the invention, this cross correlation may be derived by filtering a pilot signal.
Although decorrelation is relatively computationally intensive, it need only be performed once for a particular block of data to produce the vector uafter[n]. Since the updating of weight vectors is relatively simple, a computationally intensive RLS calculation need not be performed once for each of the users, decreasing the receiver""s hardware requirements (e.g. lower MIPS for a processor, fewer gates in an integrated circuit, and/or less current in an integrated circuit).
An alternate embodiment is disclosed in which an input signal is not decorrelated but instead multiplied by an inverse of a correlation matrix to create a modified version of the input signal. In this case, the weights applied to the modified version of the input signal to derive a symbol estimate are based on a cross correlation between the input signal (unmodified) and a desired signal.